Implement bandwidth-tests in addition to latency-tests

[nroi]

So far, we have only used latency tests mainly because they don't take as much time as latency tests and because they don't cause as much stress on the remote mirrors. It would still be beneficial to have the option to run bandwidth-tests when the user desires to do so – but not in an automated manner, like every time when Flexo starts.

Perhaps we could just provide an endpoint so that the user can run an HTTP POST request with curl in order to instruct Flexo to run bandwidth-tests.